| Power allocator governor tunables |
| ================================= |
| |
| Trip points |
| ----------- |
| |
| The governor works optimally with the following two passive trip points: |
| |
| 1. "switch on" trip point: temperature above which the governor |
| control loop starts operating. This is the first passive trip |
| point of the thermal zone. |
| |
| 2. "desired temperature" trip point: it should be higher than the |
| "switch on" trip point. This the target temperature the governor |
| is controlling for. This is the last passive trip point of the |
| thermal zone. |
| |
| PID Controller |
| -------------- |
| |
| The power allocator governor implements a |
| Proportional-Integral-Derivative controller (PID controller) with |
| temperature as the control input and power as the controlled output: |
| |
| P_max = k_p * e + k_i * err_integral + k_d * diff_err + sustainable_power |
| |
| where |
| e = desired_temperature - current_temperature |
| err_integral is the sum of previous errors |
| diff_err = e - previous_error |
| |
| It is similar to the one depicted below: |
| |
| k_d |
| | |
| current_temp | |
| | v |
| | +----------+ +---+ |
| | +----->| diff_err |-->| X |------+ |
| | | +----------+ +---+ | |
| | | | tdp actor |
| | | k_i | | get_requested_power() |
| | | | | | | | |
| | | | | | | | ... |
| v | v v v v v |
| +---+ | +-------+ +---+ +---+ +---+ +----------+ |
| | S |-------+----->| sum e |----->| X |--->| S |-->| S |-->|power | |
| +---+ | +-------+ +---+ +---+ +---+ |allocation| |
| ^ | ^ +----------+ |
| | | | | | |
| | | +---+ | | | |
| | +------->| X |-------------------+ v v |
| | +---+ granted performance |
| desired_temperature ^ |
| | |
| | |
| k_po/k_pu |
| |
| Sustainable power |
| ----------------- |
| |
| An estimate of the sustainable dissipatable power (in mW) should be |
| provided while registering the thermal zone. This estimates the |
| sustained power that can be dissipated at the desired control |
| temperature. This is the maximum sustained power for allocation at |
| the desired maximum temperature. The actual sustained power can vary |
| for a number of reasons. The closed loop controller will take care of |
| variations such as environmental conditions, and some factors related |
| to the speed-grade of the silicon. `sustainable_power` is therefore |
| simply an estimate, and may be tuned to affect the aggressiveness of |
| the thermal ramp. For reference, the sustainable power of a 4" phone |
| is typically 2000mW, while on a 10" tablet is around 4500mW (may vary |
| depending on screen size). |
| |
| If you are using device tree, do add it as a property of the |
| thermal-zone. For example: |
| |
| thermal-zones { |
| soc_thermal { |
| polling-delay = <1000>; |
| polling-delay-passive = <100>; |
| sustainable-power = <2500>; |
| ... |
| |
| Instead, if the thermal zone is registered from the platform code, pass a |
| `thermal_zone_params` that has a `sustainable_power`. If no |
| `thermal_zone_params` were being passed, then something like below |
| will suffice: |
| |
| static const struct thermal_zone_params tz_params = { |
| .sustainable_power = 3500, |
| }; |
| |
| and then pass `tz_params` as the 5th parameter to |
| `thermal_zone_device_register()` |
| |
| k_po and k_pu |
| ------------- |
| |
| The implementation of the PID controller in the power allocator |
| thermal governor allows the configuration of two proportional term |
| constants: `k_po` and `k_pu`. `k_po` is the proportional term |
| constant during temperature overshoot periods (current temperature is |
| above "desired temperature" trip point). Conversely, `k_pu` is the |
| proportional term constant during temperature undershoot periods |
| (current temperature below "desired temperature" trip point). |
| |
| These controls are intended as the primary mechanism for configuring |
| the permitted thermal "ramp" of the system. For instance, a lower |
| `k_pu` value will provide a slower ramp, at the cost of capping |
| available capacity at a low temperature. On the other hand, a high |
| value of `k_pu` will result in the governor granting very high power |
| while temperature is low, and may lead to temperature overshooting. |
| |
| The default value for `k_pu` is: |
| |
| 2 * sustainable_power / (desired_temperature - switch_on_temp) |
| |
| This means that at `switch_on_temp` the output of the controller's |
| proportional term will be 2 * `sustainable_power`. The default value |
| for `k_po` is: |
| |
| sustainable_power / (desired_temperature - switch_on_temp) |
| |
| Focusing on the proportional and feed forward values of the PID |
| controller equation we have: |
| |
| P_max = k_p * e + sustainable_power |
| |
| The proportional term is proportional to the difference between the |
| desired temperature and the current one. When the current temperature |
| is the desired one, then the proportional component is zero and |
| `P_max` = `sustainable_power`. That is, the system should operate in |
| thermal equilibrium under constant load. `sustainable_power` is only |
| an estimate, which is the reason for closed-loop control such as this. |
| |
| Expanding `k_pu` we get: |
| P_max = 2 * sustainable_power * (T_set - T) / (T_set - T_on) + |
| sustainable_power |
| |
| where |
| T_set is the desired temperature |
| T is the current temperature |
| T_on is the switch on temperature |
| |
| When the current temperature is the switch_on temperature, the above |
| formula becomes: |
| |
| P_max = 2 * sustainable_power * (T_set - T_on) / (T_set - T_on) + |
| sustainable_power = 2 * sustainable_power + sustainable_power = |
| 3 * sustainable_power |
| |
| Therefore, the proportional term alone linearly decreases power from |
| 3 * `sustainable_power` to `sustainable_power` as the temperature |
| rises from the switch on temperature to the desired temperature. |
| |
| k_i and integral_cutoff |
| ----------------------- |
| |
| `k_i` configures the PID loop's integral term constant. This term |
| allows the PID controller to compensate for long term drift and for |
| the quantized nature of the output control: cooling devices can't set |
| the exact power that the governor requests. When the temperature |
| error is below `integral_cutoff`, errors are accumulated in the |
| integral term. This term is then multiplied by `k_i` and the result |
| added to the output of the controller. Typically `k_i` is set low (1 |
| or 2) and `integral_cutoff` is 0. |
| |
| k_d |
| --- |
| |
| `k_d` configures the PID loop's derivative term constant. It's |
| recommended to leave it as the default: 0. |
| |
| Cooling device power API |
| ======================== |
| |
| Cooling devices controlled by this governor must supply the additional |
| "power" API in their `cooling_device_ops`. It consists on three ops: |
| |
| 1. int get_requested_power(struct thermal_cooling_device *cdev, |
| struct thermal_zone_device *tz, u32 *power); |
| @cdev: The `struct thermal_cooling_device` pointer |
| @tz: thermal zone in which we are currently operating |
| @power: pointer in which to store the calculated power |
| |
| `get_requested_power()` calculates the power requested by the device |
| in milliwatts and stores it in @power . It should return 0 on |
| success, -E* on failure. This is currently used by the power |
| allocator governor to calculate how much power to give to each cooling |
| device. |
| |
| 2. int state2power(struct thermal_cooling_device *cdev, struct |
| thermal_zone_device *tz, unsigned long state, u32 *power); |
| @cdev: The `struct thermal_cooling_device` pointer |
| @tz: thermal zone in which we are currently operating |
| @state: A cooling device state |
| @power: pointer in which to store the equivalent power |
| |
| Convert cooling device state @state into power consumption in |
| milliwatts and store it in @power. It should return 0 on success, -E* |
| on failure. This is currently used by thermal core to calculate the |
| maximum power that an actor can consume. |
| |
| 3. int power2state(struct thermal_cooling_device *cdev, u32 power, |
| unsigned long *state); |
| @cdev: The `struct thermal_cooling_device` pointer |
| @power: power in milliwatts |
| @state: pointer in which to store the resulting state |
| |
| Calculate a cooling device state that would make the device consume at |
| most @power mW and store it in @state. It should return 0 on success, |
| -E* on failure. This is currently used by the thermal core to convert |
| a given power set by the power allocator governor to a state that the |
| cooling device can set. It is a function because this conversion may |
| depend on external factors that may change so this function should the |
| best conversion given "current circumstances". |
| |
| Cooling device weights |
| ---------------------- |
| |
| Weights are a mechanism to bias the allocation among cooling |
| devices. They express the relative power efficiency of different |
| cooling devices. Higher weight can be used to express higher power |
| efficiency. Weighting is relative such that if each cooling device |
| has a weight of one they are considered equal. This is particularly |
| useful in heterogeneous systems where two cooling devices may perform |
| the same kind of compute, but with different efficiency. For example, |
| a system with two different types of processors. |
| |
| If the thermal zone is registered using |
| `thermal_zone_device_register()` (i.e., platform code), then weights |
| are passed as part of the thermal zone's `thermal_bind_parameters`. |
| If the platform is registered using device tree, then they are passed |
| as the `contribution` property of each map in the `cooling-maps` node. |
| |
| Limitations of the power allocator governor |
| =========================================== |
| |
| The power allocator governor's PID controller works best if there is a |
| periodic tick. If you have a driver that calls |
| `thermal_zone_device_update()` (or anything that ends up calling the |
| governor's `throttle()` function) repetitively, the governor response |
| won't be very good. Note that this is not particular to this |
| governor, step-wise will also misbehave if you call its throttle() |
| faster than the normal thermal framework tick (due to interrupts for |
| example) as it will overreact. |
